docs: def _get_phi_al_regional(self, C, mag, vs30measured, rrup):
          """
          Returns intra-event (Phi) standard deviation (equation 24, page 1046)
          """
          phi_al = np.ones((len(vs30measured)))
          s1 = np.ones_like(phi_al) * C['s1e']
          s2 = np.ones_like(phi_al) * C['s2e']
          s1[vs30measured] = C['s1m']
          s2[vs30measured] = C['s2m']
          if mag < 4:
              phi_al *= s1
          elif mag <= 6:
              phi_al *= s1 + (s2 - s1) / 2. * (mag - 4.)
          else:
              phi_al *= s2
          return phi_al